This invention relates to a diaphragm control device for a camera, and more specifically to a diaphragm control device using a stepping motor.
With the automation of the exposure control of cameras, cameras have recently been provided with diaphragm control devices. A diaphragm mechanism of a camera has a plurality of diaphragm leaves interlocking with one another to vary the size of a diaphragm opening. A linking pin is attached at right angles to one of these diaphragm leaves, and the diaphragm opening is changed in size by turning the linking pin around the optical axis of the camera. Conventionally, there has been proposed a diaphragm control device to control the angle of rotation of the linking pin of the diaphragm mechanism by using a stepping motor. In such device, a clock pulse corresponding to the brightness of a subject is supplied to the stepping motor to rotate the motor stepwise. A stopper lever is fixed on a shaft of the stepping motor. The diaphragm mechanism is so designed as to be urged in one direction to open or close the opening in response to a depression of e.g. a release button prior to a shutter operation. The size of the diaphragm opening is determined when the turn of the linking pin is prevented by the stopper lever in the middle of the urging of the diaphragm mechanism in the other direction.
After the shutter operation is finished, the diaphragm mechanism and the stepping motor must be restored to their respective reference positions. In an initial excitation state, the stepping motor has a plurality of stop positions provided by attraction and repulsion between electromagnets serving as a stator and permanent magnets arranged on a rotor. Although the diaphragm mechanism may easily be restored to its reference position by a spring, etc., the stepping motor cannot have its reference position fixed by the biasing force of a spring or the like. The reason is that the stepping motor cannot rotate stepwise against such biasing force because of its weak turning force. Accordingly, there has been proposed a system to rotate the stepping motor in the reverse direction by applying a pulse of opposite polarity thereto and to stop the rotation at the reference position by using an optical detector. With such system, however, the device cannot be reduced in size as well as in cost. Moreover, the detector will require additional power consumption. Alternatively, there is used a system in which the number of clock pulses supplied for the rotation of the stepping motor is previously stored, and the stepping motor is restored to the reference position by reversely applying pulses of such number to the stepping motor. This system, however, still requires an expensive memory device.
The object of this invention is to provide a diaphragm control device for a camera having a simple construction and capable of setting a diaphragm opening in accordance with the brightness of a subject and returning to a reference position after the performance of an operation of the camera and before the setting of the diaphragm opening for the next camera operation.